Mixtures of iron oxide, oxides of transition metals, other metals, and semi-metals are basic components in the manufacture of many products that are used for electromagnetic interference (EMI) suppression filters, inductors, and reprographic system components, among others. Ferrite materials, commonly called spinel, may be produced by forming small particles of chemical oxides. Other crystal structures, such as garnet, and hexagonal ferrite, may be also be produced. Such spinel ferrites may be based on an iron oxide denoted by the formula, MeFe2O4, and may contain as the Me element, for example, some combination of substituted transition metals such as manganese (Mn), nickel (Ni), cobalt (Co), vanadium (V), as well as other oxides of metals such as magnesium (Mg), copper (Cu), aluminum (Al), and zinc (Zn).
These ferrites may also contain semi-metals such as silicon (Si), and other additives such as titanium (Ti), tantalum (Ta), niobium (Nb), vanadium (V), and even alkaline earth metals such as calcium (Ca). In some cases, alkali metals may reside in the ferrite or in other phases.
Although referred to as spinel, garnet and hexagonal ferrite, these materials may be complex multiphase materials containing phases such as FeO and glass formers that are used to control bulk electrical resistivity, eddy currents, frequency response of material impedance, magnetic hysteretic characteristics, total magnetic moment, and sintering characteristics.
The production of ferrites may be controlled intentionally to produce spinels of oxides of iron and other elements, which may occur in more than one valence state having controlled ferrimagnetic properties. In many applications a particular surface morphology and a specific size of powder may be required to achieve desired product properties.
To obtain these desired product properties in a final product shape, a ferrite, or precursor mixture, must first be produced. This may be accomplished by methods well known to persons experienced in the technology and may include processes such as chemical precipitation, the use of naturally occurring oxide ores, or conversion of aqueous solution of metal salts such as chlorides, and even melting the starting ingredients.
The overall elemental composition of the incipient ferrite spinel may be created by mixing exact proportions of metal oxides, or chemical precursors. The overall elemental composition may require grinding elemental oxides in a proper proportion into an intimate mixture of small particle size, and adjusting the composition of the final mixture, which may then be spray dried.
Such mixtures may be subjected to intermediate thermal treatments in rotary or fixed kilns to partially react them, or to produce particle sizes useful for subsequent mechanical processing. After thermal-mechanical processing, the materials may be ground again into small particle size, and the composition may be adjusted to meet a target terminal composition. The materials may thereafter be spray dried in an aqueous process to create an aggregate that may be mixed and blended, pressed into a shape, and sintered.
For some applications, such as reprographic use, the powder may be used in the spray dried and sintered form. In other applications, the powder may be used with, or without, spray drying when it is intended as an additive to a mixture of an inorganic or organic binder.
A process for producing a pre-reacted oxide powder is disclosed in U.S. Pat. No. 5,976,488 to Workman et al., the entire disclosure of which is incorporated herein by reference. While this process produces pre-reacted powder, its particular phase mixture is not typically suitable for direct use in ferrite products without further processing and thermal treatments. Moreover, the atmospheric conditions used to produce such powders may not be effectively controlled to achieve a desired oxidation state and phase composition of all the iron and other elements.
The process described in the Workman et al. patent produces particle morphology that is somewhat useful for carrier bead, but the ratio of ferri-magnetic spinel to non-ferrimagnetic oxides of iron and other elements leads to a magnetic moment that may be too low for use. Further, the ratio may also lead to a volume electrical resistivity that is not suitable. Accordingly, this product must be further processed to produce a useful carrier bead.
Additionally, the process described in the Workman et al. patent may produce detrimental particle shapes, such as broken particles with sharp edges and elongate particles. It may be difficult and costly to remove these irregular particles by traditional separation methods since they exist in a range of sizes. Further, a significant percentage of useful spherical powder may be lost when conventional separation methods are used. A negative economic impact is observed due to a small percentage of total powder that is usable.